How old is my Williams furnace?

Serial number age decoder for Williams furnaces: 2 format eras, the decode rule in plain English, transcribed tables, and the official lookup.

Serial number decoder for Williams furnace

Serial no.

Type the serial from the data plate, not the model number. Nothing is sent anywhere; the tables are in this page.

Format example: 13 AD

Williams furnace serial numbers in the 1984 to 2004 format encode the year as letter 3 (table below) and the month as letter 2 (A=Jan..M=Dec, I skipped); day as digits 1-2 (source: building-center.org).
Example: serial 13 AD decodes to January 1987.
Williams's official lookup at williamscomfort.com returns warranty / registration lookup.

Format eras

1984 to 2004 VERIFIED
## AAletter 3: A=1984 B=1985 C=1986 D=1987 E=1988 F=1989 G=1990 H=1991 J=1992 K=1993 L=1994 M=1
era not stated PARTIAL
no date in serialdigits 3-4 (YY) -> 2005

How to read the serial

1984 to 2004 formatVERIFIED

Shape ## AA · example 13 AD · confidence medium

Year: letter 3: A=1984 B=1985 C=1986 D=1987 E=1988 F=1989 G=1990 H=1991 J=1992 K=1993 L=1994 M=1995 N=1996 P=1997 R=1998 S=1999 T=2000 V=2001 X=2002 Y=2003 Z=2004 (I, O, Q, U, W skipped)
Week or month: month = letter 2 (A=Jan..M=Dec, I skipped); day = digits 1-2

Year code	Year(s)
A	1984
B	1985
C	1986
D	1987
E	1988
F	1989
G	1990
H	1991
J	1992
K	1993
L	1994
M	1995
N	1996
P	1997
R	1998
S	1999
T	2000
V	2001
X	2002
Y	2003
Z	2004

Month code	Month
A	January
B	February
C	March
D	April
E	May
F	June
G	July
H	August
J	September
K	October
L	November
M	December

Example: serial 13 AD decodes to January 1987.

Trap: Date-code sticker, separate from the data plate. Letters after 2004 not charted (a new cycle presumably started). Example = 13 Jan 1987.

era not stated formatPARTIAL

Year: digits 3-4 (YY) -> 2005
Week or month: week = digits 1-2 -> week 27

Documented but not evaluated by the decoder: no example serial.

Trap: No real example: building-center's own sample is masked (2705#####), so example is null. Same WWYY shape as Carrier style 1 minus the plant letter; 9-digit numeric collides broadly. Digit count after WWYY is inferred from the mask.

Where the data plate is on a furnace

Inside the furnace: remove the upper (burner) access door and look on the inside of the door, on the blower compartment wall, or on a sticker facing you at the burner deck.

General guidance, not brand-specific: the owner's manual or the installer can point to the exact spot. See the data plate guide.

For scale only: the InterNACHI Standard Estimated Life Expectancy Chart for Homes lists Furnace at 15 to 25 years of expected service (nachi.org). That chart is a guideline, not a prediction for your unit.

Official lookup: the authority

Open the official Williams lookup takes serial number; returns warranty / registration lookup; URL verified live

This site is a reference built from published tables. The manufacturer's own lookup wins whenever the two disagree.

What we could not confirm

The era not stated format is only partially confirmed (PARTIAL (masked example)).
The era not stated format is documented but not decodable here: no example serial.
Style 1 is a separate DATE-CODE STICKER (DD + month letter + year letter), not the serial. Style 2 serial = WWYY#####. Year-letter chart ends at Z=2004 (page last updated 2013). williamscomfort.com/warranty-v2-temp/ = 200 (warranty claim form); williamscomfortprod.com redirects there.

Sources

https://www.building-center.org/williams-hvac-age/
img: WILLIAMS-Date-of-Manufacture-Serial-Number-Style-1.jpg (transcribed)
img: WILLIAMS-...-Style-2.jpg
https://williamscomfort.com/warranty-v2-temp/official